Power-operated boosters for vehicle braking systems

Abstract

In a power-operated hydraulic booster for a vehicle braking system pressurization of a power chamber in a housing on one side of a boost piston is controlled by an assembly of at least two separate spool parts which work in bores in the boost piston and the housing, and a boost chamber is defined in the housing between the power chamber and an output piston having an effective area less than that of the boost piston.

We claim: 
     
       1. A power-operated hydraulic booster for a vehicle braking system comprising a housing having an inlet port for connection to a source of hydraulic fluid under pressure, an outlet port for connection to a reservoir for hydraulic fluid and a bore, a booster piston adapted to work in said bore and defining a power chamber in said bore, and valve means controlling communication between said ports and said power chamber, said valve means being so constructed and arranged that in response to an input force hydraulic fluid from said inlet port pressurises said power chamber to advance said boost piston in said housing thereby augmenting said input force, and said valve means comprises a spool working in complementary bores in said boost piston and an adjacent end of said housing, said spool comprising an assembly of at least two separate freely separable uncoupled spool parts having adjacent ends in abutment and provided with passage means which communicate at said adjacent ends, and sealing means are disposed between said adjacent ends to seal said passage means from said bores at least at said adjacent ends, one of said spool parts working in said bore in said boost piston and the other of said spool parts working in said bore in said housing. 
     
     
       2. An hydraulic booster as claimed in claim 1, wherein said end of said one spool part is received within a recess in said end of the said other spool part and said recess is of a diameter greater than that of the said one spool part, said sealing means comprising a seal provided between interengaging faces of the spool part ends. 
     
     
       3. An hydraulic booster as claimed in claim 2, wherein said seal comprises an annular seal located in said end of the said one spool part for engagement with said face which defines the base of said recess. 
     
     
       4. An hydraulic booster as claimed in claim 1, wherein said booster is of the "closed-centre" type in which said valve means normally cut-off communication between said power chamber and said inlet port and provide communication between said power chamber and said outlet port and, when said booster is operated in response to said input force, said valve spool is operated initially to cut-off communication between said power chamber and said outlet port and subsequently to place said power chamber in open communication with said inlet port so that hydraulic fluid under pressure is admitted to and pressurises said power chamber. 
     
     
       5. An hydraulic booster as claimed in claim 1, wherein said booster of the "open-centre" type in which hydraulic fluid is normally circulated continuously between said inlet and outlet ports through communicating passages in said booster piston and said valve spool and through said power chamber and, when said booster is operated, said valve spool is operative to cut-off communication between said power chamber and said outlet port so that pressure from said inlet port can build up in and pressurise said power chamber. 
     
     
       6. A power-operated hydraulic booster for a vehicle braking system comprising a housing having an inlet port for connection to a source of hydraulic fluid under pressure, and outlet port for connection to a reservoir for hydraulic fluid and a bore, a boost piston adapted to work in said bore and defining a power chamber in said bore, valve means controlling communication between said ports and said power chamber, said valve means being so constructed and arranged that in response to an input force hydraulic fluid from said inlet port pressurises said power chamber to advance said boost piston in said housing thereby augmenting said input force, an output piston adapted to work in said bore in a position in advance of said boost piston on the opposite side of said boost piston from said power chamber to define a boost chamber between boost piston and said output piston, said valve means also being so constructed and arranged to control communication between said power chamber and said boost chamber, and a connection between said chambers, said connection being normally open but, when said valve means are operated in response to said input force, is closed to isolate said chambers from each other before said power chamber is pressurised whereby said boost piston acts on said output member through a volume of fluid trapped in said boost chamber. 
     
     
       7. A power-operated hydraulic booster as claimed in claim 6, wherein said valve means comprises a valve spool working at least in part in a bore in said boost piston to control pressurisation of said power chamber. 
     
     
       8. A power-operated hydraulic booster as claimed in claim 7, wherein said valve spool controls communication between complementary passages in said boost piston and said housing. 
     
     
       9. A power-operated hydraulic booster as claimed in claim 7, wherein a spring-loaded axially movable valve member for engagement with a seating surrounding an opening between said bore in the boost piston and said boost chamber is coupled to the inner end of said valve spool for relative axial movement, the arrangement being such that said valve member is engageable with said seating to isolate the chambers before said valve spool permits said power chamber to be pressurised. 
     
     
       10. A power-operated hydraulic booster as claimed in claim 6, wherein the relative areas of said boost piston and said output piston can be varied to vary, as desired, the degree of travel of said output piston and the effective volume of said boost chamber. 
     
     
       11. A power-operated hydraulic booster as claimed in claim 6, incorporating an input piston working in an output bore in said housing which communicates at its inner end with said power chamber, whereby the relative areas of said input piston and said power-piston can be varied to vary, as desired, the magnitude of said input force and the travel of said input piston required to operate said booster. 
     
     
       12. A power-operated hydraulic booster as claimed in claim 11, wherein said valve means comprises a valve, spool of which at least a part is integral with said input piston. 
     
     
       13. In combination for a vehicle hydraulic braking system: a power-operated hydraulic booster and an hydraulic master cylinder wherein said booster comprises a housing having an inlet port for connection to a source of hydraulic fluid under pressure, an outlet port for connection to a reservoir for hydraulic fluid and a bore, a boost piston adapted to work in said bore and defining a power chamber in said bore, valve means controlling communication between said ports and said power chamber, said valve means being so constructed and arranged that in response to an input force hydraulic fluid from said inlet port pressurises said power chamber to advance said boost piston in said housing thereby augmenting said input force, and said valve means comprises a spool working in complementary bores in said boost piston and an adjacent end of said housing, said spool comprising an assembly of at least two separate spool parts which are in abutment at adjacent ends, and sealing means are disposed between said ends, one of said spool parts working in said bore in said boost piston and the other of said spool parts working in said bore in said housing, an output piston adapted to work in said bore in a position in advance of said boost piston on the opposite side of said boost piston from said power chamber to define a boost chamber between boost piston and said output piston, said valve means also being so constructed and arranged to control communication between said power chamber and said boost chamber, and a connection between said chamber, said connection being normally open but, when said valve means are operated in response to said input force, is closed to isolate said chambers from each other before said power chamber is pressurised whereby said boost piston acts on said output member through a volume of fluid trapped in said boost chamber, and wherein said master cylinder is provided with an operating rod which projects into said housing of said booster and is acted upon by said output piston. 
     
     
       14. The combination as claimed in claim 13, wherein said output piston acts on the operating rod through a thrust assembly of which the effective length is adjustable to compensate for axial tolerance variations.